Because an electron bears a negative charge and has a very small mass, electron beams in which electrons are arranged to travel in one direction have the following special features. (1) The direction and the degree of convergence can be controlled by an electric or magnetic field. (2) Energy in a wide range can be obtained by acceleration and deceleration with an electric field. (3) Because the wavelength is short, the beam can be converged to a small diameter. Electron microscopes and electron beam exposure devices employing such special features have been widely used. As cathode materials for such devices, cheap W filaments or hexaborides, e.g., LaB6 which can produce an electron beam with a high brightness, are exemplified for thermal electron emission sources. Further, W with a sharpened tip that uses a tunnel phenomenon based on a quantum effect and ZrO/W using the Schottky effect based on electric filed application have been used as cathodes with a high brightness and a narrow energy width.
However, although W filaments are inexpensive, a problem associated therewith is that the service life thereof is extremely short (about 100 hours). The resultant problem is that a replacement operation such as opening a vacuum container to the atmosphere or adjusting the optical axis of electron beam when the filament is broken has to be frequently performed. The service life of LaB6 is about 1000 hours and longer than that of W filaments, but because it is used in devices in which beams with a comparatively high brightness are obtained, the replacement operation is most often performed by the device manufacturers and the cost thereof is high. A problem associated with ZrO/W that has a comparatively long service life (about one year) and W with a sharpened tip that allows a higher brightness to be obtained is that the replacement cost is high.
Because electron microscopes are presently required to enable highly accurate observations of even smaller objects and because the development of electron beam exposure devices advanced to a node size of less than 65 nm, the cathodes with even higher brightness and narrower energy width are needed.
Diamond is a material that meets such expectations. Diamond exists in a state with a negative electron affinity (NEA) or in a state with a positive electron affinity (PEA) less than that of metals with a small work function, as described in Non-Patent Document 1 and Non-Patent Document 2. By employing such an extremely rare physical property, it is possible to obtain electron emission with a high current density and to reduce the energy width, without requiring a high temperature in excess of 1000° C. as in W filaments, LaB6, or ZrO/W. Further, because the operation temperature is low, a long service life can be expected. In addition, because there is a microprocessing technology that can produce a tip end diameter of 10 nm, as described in Non-Patent Document 3, no problems are associated with increase in brightness. Since diamond has been found to have the aforementioned electron affinity, electron sources as described in Non-Patent Document 4 and Patent Document 1 have been proposed.    Non-Patent Document 1: F. J. Himpsel et al., Phys. Rev. B., Vol. 20, Number 2 (1979), 624-    Non-Patent Document 2: J. Ristein et al., New Diamond and Frontier Carbon Technology, Vol. 10, No. 6, (2000) 363-    Non-Patent Document 3: Y. Nishibayashi et al., SEI Technical Review, 57, (2004) 31-    Non-Patent Document 4: W. B. Choi et al., J. Vac. Sci. Technol. B14 (1996) 2051-    Patent Document 1: Japanese Patent Publication No. 4-67527A